Net molded tantalum carbide rocket nozzle throat

ABSTRACT

A method of making heat engine components, such as rocket nozzle throats, includes forming a body having at least one surface, wherein the body is made of a refractory metal, and compressing a ceramic powder between a die and the at least one surface of the body, with sufficient heat and pressure to densify and thus form a solid ceramic coating on the at least one surface of the body. Nozzle throats made according to the invention have bodies made of refractory metals, such as tantalum, and ceramic coatings on the inner surfaces of the annular bodies, wherein the ceramic coatings are made of ceramic materials such as tantalum carbides (TaC and Ta 2 C).

BACKGROUND OF THE INVENTION

The present invention relates generally to metal/ceramic compositestructures and methods of making same, and more specifically, to atantalum-carbide ceramic nozzle throat made by diffusion bondingtantalum-carbide/tantalum₂-carbide to tantalum. In particular, a nozzlethroat is made by forging a ceramic layer to a tantalum metal ring athigh temperatures and pressures.

DESCRIPTION OF THE RELATED ART

Materials used for rocket nozzle throats, such as those used in tacticaland strategic missiles, must survive severe thermal environments foranywhere from two seconds to over one minute. In previous long-rangemissiles, performance was the primary design driver. High performancegoals led to the usage of expensive, exotic materials in theconstruction of the nozzle. One key component of the nozzle is thenozzle throat section, where the environment is most severe. Mostlong-range missiles in use today use carbon-carbon material.Carbon-carbon performs adequately but it is highly labor intensive,often taking up to two years to deliver one nozzle throat section, thusrendering the nozzle throat very expensive to produce. Less expensivematerials have been fabricated into nozzle throats but failed to performas well as carbon-carbon.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a rocket nozzle throatthat can be manufactured at substantial cost savings relative tocarbon-carbon throats. This is achieved by the method of making rocketnozzle throats in which a ceramic material is net molded and bonded to ametallic liner, thereby reducing its manufacturing time compared to thatof carbon-carbon.

Another object of the present invention is to provide a rocket nozzlethroat that has better performance, in terms of erosion, than that ofcarbon-carbon, it being understood that lower erosion rates lead toimproved missile range.

These and other objects of the invention are met by providing a rocketnozzle throat that includes a refractory metal ring and a ceramic layernet molded and diffusion bonded to the inner surface of the metal ring.Preferably, the ceramic is made of a mixture of tantalum carbide andtantalum₂ carbide (TaC/Ta₂C), here after referred to as tantalumcarbide, and the metal ring is made from tantalum (Ta).

The refractory metal ring is further preferably made from a materialselected from the group consisting of tantalum, tantalum alloys,molybdenum alloys, hafnium alloys, titanium alloys, tungsten alloys, andniobium alloys. Examples of suitable tantalum alloys include Ta—10W,Ta—2.5W, and Ta—40Nb. Another alloys that could be used is “C103,” whichis 89% Nb, 10% Hf, and 1% Ti.

These and other objects of the invention will become more apparent fromthe following detailed description when taken in conjunction with theillustrative embodiments in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a horizontal cross-section of a rocket motor of the generaltype used in tactical missiles, shown for illustrative purposes but notto scale;

FIG. 2 is an enlarged view, partially cut-away, showing the rocketnozzle throat according to the present invention;

FIGS. 3 through 6 illustrate schematically, in sequence, the process ofmaking rocket nozzle throats according to a preferred embodiment of thepresent invention;

FIG. 7 is a photograph showing the diffusion-bonded microstructure of arocket nozzle throat;

FIG. 8 is a photograph showing the ceramic throat after rocket test;

FIG. 9 is a detail drawing of the nozzle shown in 8, illustrating thethree inserts that compose the throat section 20;

FIG. 10 is a cross-sectional view of a larger scale nozzle throat;

FIG. 11 are post-test photographs of the inserts shown in FIG. 10; and

FIG. 12 is a cross-sectional view of a single piece nozzle throatdesign.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a rocket motor 10 includes a body 12 which definesan interior chamber. The chamber contains a solid rocket propellant 16.A forward end 14 of the body 12 is closed, while a nozzle 18 is disposedat the opposite end. A throat insert 20 is disposed in a constrictedzone of the nozzle 18. Combustion gases from the propellant pass throughan opening of the nozzle throat 20 before expanding and exiting thenozzle 18.

Referring to FIG. 2, the throat insert 20 includes a refractory metalring 22 having a ceramic layer 24 diffusion bonded to the inner surfaceof the refractory metal ring. The refractory metal ring 22 is generallyin the shape of an annulus having a central passageway. A throat regionof the passageway is defined by circular, convex surface that defines aconvergence/divergence region through which pass the expanding exhaustgases of the propellant.

The process of making a rocket nozzle throat, which may be in the formof an insert, is illustrated in FIGS. 3 through 6. The refractory metalring 22 shown in FIG. 3 is first machined into desired shape, such as anannulus with its inner surface defined by a conical surface. A preferredmaterial from which to make the ring 22 is tantalum, although similarrefractory metals could be used. Other refractory metals that could beused include tantalum alloys, niobium alloys, hafnium alloys, molybdenumalloys, titanium alloys and tungsten alloys. Some examples of tantalumalloys (listed in weight percentages) include Ta—2.5W, Ta—10W, andTa—40Nb. Another example of suitable alloys includes “C103,” whichincludes 89% niobium, 10% hafnium, and 1% titanium. Selection of themetal would be based on refractory-ness and high temperature compliance.Moreover, the material must be able to withstand the thermo-mechanicalloading conditions imposed upon it by the ceramic material. Thecomposite system must demonstrate the ability to diffusion bond.

Other ceramic materials can include ZrC, HfC, and NbC. Selection of theceramic material would be based on refractory-ness (melting point) andfracture toughness. Other properties such as density, hardness, thermalproperties, etc., would be relevant for design purposes; however, theywould not be limiting factors.

The metal ring is then placed inside of a forging tool 30, FIG. 4,having a central mandrel 31. The ceramic powders are placed in theforging tool, and a forging die 28 is placed over the powders. FIG. 4shows the pre-pressed state of the operation. As seen in FIG. 5, at hightemperatures, the forging tool 28 is pressed to consolidate the powdersand to create a bond between the refractory metal 22 and the ceramicmaterial.

The forging process takes place at extreme temperatures and pressures,e.g., temperatures at or above 3,000° F., and preferably between 3,000and 5,000° F., and pressures at or above 10,000 psi, and preferablybetween ½ to 15 ksi.

In general, powder consolidation is expected to occur in the preferredrange of 3,000 and 5,000° F., which are typically temperatures greaterthan ½ the melting point of the ceramic, yet less than the melting pointof the metal. The applied pressure depends to some degree on the samplegeometry; for example, a lower aspect ratio requires lower pressure.These temperatures and pressures are required to obtain a dense ceramiccompact which demonstrates a cohesive interface with the refractorymetal.

The powders are packed in the inner portion of the metal ring 22, andare subjected to the aforestated temperatures and pressures to therebynet-mold and bond the ceramic material to the metal ring. After initialapplication of pressure, the ceramic is allowed to cool down underpressure. During this step, the ceramic throat achieves a packingdensity of 95% maximum theoretical density. The molding step anddensification of the ceramic material is shown in FIG. 5, whereby theceramic material conforms to the shape of the inner surface of therefractory metal ring 22. As a final step, the throat insert undergoes aminimal amount of machining to make the insert match designspecifications, thus completing the throat insert fabrication process.FIG. 6 shows one example of the finished product.

The diffusion bond formed between the ceramic material and the metallicmaterial, produced at these extreme temperatures and pressures, reducesstresses due to thermal shock during motor firing. A microscopic photoof the diffusion bond is shown in FIG. 7. The darker portion of thephoto is the ceramic layer 24. The lighter and more defined structure isthe refractory metal ring 22. In between is the region of diffusion bond33 where a transition from ceramic to metal can be observed.

The single step molding of the ceramic to metal ring dramatically lowersprocessing time of a nozzle throat compared to that of labor-intensivecarbon-carbon.

The process described for making rocket nozzle throats, by bondingtantalum-carbide to a tantalum ring, provides a nozzle throat thatbetter withstands thermal shock and erosion than previous ceramic nozzlethroats. Static-fire test results have demonstrated better erosioncharacteristics than that of carbon-carbon; this has the potential tolead to better missile performance during flight. FIG. 8 is a photographof one such experiment. Throat insert 20 is visible from the aft end ofthe experimental nozzle. The thickness of the ceramic layer is chosen byconsidering bum time and required ablation depth of ceramic, nozzlethroat contour requirements, and ablation rates of surroundingmaterials.

In one example of a throat insert, the refractory metal ring has a widthof 1.5 inches, an inner diameter of 5.0 inches and an outer diameter of6.0 inches. The metal ring can be machined from a 0.5-inch thick flatplate. The ceramic powders are mixed and packed on top of the innerannulus surface. The forging rams press the powders under hightemperature and pressure, molding the ceramic into a tapered shape. Thethickness of ceramic can be as much as 0.5″. The final machining stepfinishes the throat insert into the desired shape design specificationsfor final assembly.

While the description shows a particular shape of the ceramic throat,virtually any type and shape of throat is within the scope of thepresent invention. Moreover, while the invention has been described withrespect to rocket nozzle throats, other types of products can be madeusing the manufacturing techniques.

Although the invention has been described with reference to a particularembodiment, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thesprit of the appended claims.

What is claimed is:
 1. A rocket nozzle venturi throat comprising: atantalum refractory metal ring having a generally annular shape and acentral passageway defined by an inner surface; and a ceramic layerdiffusion bonded to the inner surface of the metal ring, the ceramiclayer being formed of a material consisting essentially of a carbide,wherein the carbide is a composition of tantalum carbide and tantalum₂carbide; and a diffusion bond region comprising a transition between therefractory metal ring and the ceramic layer.
 2. A rocket motorcomprising: a body having a first closed end and a second end anddefining a chamber for containing propellant; a nozzle connected to thesecond end of the body, and being in communication with the chamber; anda venturi throat disposed within the nozzle, wherein the venturi throatincludes a tantalum refractory metal ring having a generally annularshape and a central passageway defined by an inner surface, and aceramic layer diffusion bonded to the inner surface of the metal ring,the ceramic layer being formed of a material consisting essentially of acarbide, wherein the carbide is a composition of TaC and Ta₂C, and adiffusion bond region comprising a transition between the refractorymetal ring and the ceramic layer.
 3. A rocket nozzle venturi throatcomprising: a refractory metal ring having a generally annular shape anda central passageway defined by an inner surface; and an annular ceramiclayer formed on the inner surface of the metal ring by forging attemperatures and pressures sufficient to create a diffusion bond betweenthe ceramic layer and the refractory metal ring, the annular ceramiclayer being formed in situ in near net shape and being made of amaterial consisting essentially of carbides, wherein the carbides are amixture of TaC and Ta₂C.